Bind process to network interface

Question

I'm attempting to perform simultaneous bandwidth tests from a single machine using multiple network interfaces. None of the utilities I've found so far allow me to do this. The closest was iperf which has the '-B' option which allows me to bind to an ip address, but does not actually use both interfaces.

For instance let's say wlan0 has been assigned 192.168.1.100 and wlan1 is assigned 192.168.1.101. When I specify that one instance of iperf uses .100 and the other .101, only wlan0 will be used. I get similar results when I use the bind.so shim that has been posted on this site.

The iperf servers will claim that they are receiving traffic from both the .100 and .101 addresses, but like I said, only wlan0 is being utilized.

I know that only wlan0 is being utilized because: a)The activity light on only one of the wifi adapters will be blinking.
b) My spectrum analyser confirmed that only one of the adapters was generating rf in the 2.4GHz spectrum.

Using the ping command, and specifying the interface, packet size, and interval allowed me to confirm that it is possible to get both of the interfaces working simultaneously and transmitting reasonable amounts of data.

So what I am looking for is a utility that will measure point to point bandwidth and allows me to specify the network interface it is communicating on or a way to bind a process to an interface (not ip address)

Currently I'm running slackware 14.0 on my box. Ubuntu ended up being a pain in the ass because it was trying to do everything for me and ended up making the task more difficult.